1. Field of the Invention:
The present invention relates to an inverter device that produces multiple-phase alternating-current power from direct-current power and to a drive unit using the inverter device.
2. Description of Related Art:
Conventionally, an electric power steering system for assisting steering, which is performed by a driver, has been publicly known. An inverter device that produces alternating-current power from direct-current power is used as a controller for controlling drive of a motor in the electric power steering system. The inverter device is constructed by mounting electronic components such as semiconductor power devices such as transistors, resistors and capacitors on a power substrate. For example, a control unit and an electric power converter that are applied to an electric power steering controller and that correspond to an inverter device are described in Patent document 1 (JP-A-2004-237832) and Patent document 2 (JP-A-2008-029093).
In the control unit for the electric power steering described in Patent document 1, a power substrate (drive substrate) and a control board are arranged to be perpendicular to each other. However, the power substrate is formed in a single planar shape, on which multiple power elements (semiconductor switching elements) are mounted. Therefore, heats generated by adjacent power elements interfere with each other, thereby hindering heat dissipation. Therefore, there is a possibility that a failure or malfunction due to temperature increase of the power elements is caused.
In the electric power converter described in Patent document 2, one unit (power switching device group) is constituted for each set of bridge circuit. For example, a bridge circuit for producing three-phase alternating current has three units. When two systems of circuits for producing the three-phase alternating current are used, six units are used, thereby increasing the number of units.